1. Field of the Invention
The present invention relates to ultrasonic emitters, and more particularly to ultrasonic emitters which include impedance matching structure such as acoustic transformers having a horn configuration.
2. Prior Art
A variety of emitter devices have been developed which propagate ultrasonic energy. These include piezoelectric transducers, electrostatic emitters, mechanical drivers, etc. A challenge with the use of such devices in air is to provide impedance matching methods to enhance the efficiency of power transfer to the ambient air. For example, the wave impedance of a piezoelectric material such as barium titanate exceeds that of air by a factor of 105. This extreme impedance difference severely attenuates transmission into a propagated ultrasonic beam of energy into the air.
The use of acoustic horns as transformer devices is well known with respect to most sound systems for both audio and ultrasound frequencies. Extensive research has been done detailing preferred horn configurations for specific frequency ranges. Mathematical formulas are generally available to optimize the geometry of each application for a given frequency.
A publication by Fletcher and Thwaites entitled “Multi-horn Matching Plate for Ultrasonic Transducers” Ultrasonics 1992, Vol 30, No. 2., discloses the use of an array of acoustic horns formed in a plate as an acoustic transformer for ultrasonic transmission into air. Based on this disclosure, FIG. 1 shows a transducer aligned with a horn plate. A spacing gap between the emitter element and throats of the respective horns is illustrated and identified as a key element in optimizing the efficiency of the horn array for ultrasonic energy. By choosing a gap distance specifically selected for a given horn array, the publication suggests improvement of pressure gain in transducer output by 10 dB or better.
Despite enhancement of the effectiveness by this horn array system, there remain significant problems in impedance matching, particularly with ultrasonic emitters.
Many new applications of ultrasonic energy, including parametric speakers, are offering new opportunities which require high levels of efficiency in order to get a commercially acceptable audio output from ultrasonic emissions. Generally, these parametric applications depend on effective impedance matching to enable propagation of ultrasonic waves into the air as the nonlinear medium necessary for acoustic heterodyning.